1. Field of the Invention
The present invention relates to a discharge counter to ascertain the operation of a lightening arrester to protect a power system and a electric apparatus from overvoltage.
The present invention further relates to a nonlinear resistance element developed for the discharge counter.
2. Description of the Related Art
In general, an overvoltage protection instrument such as an arrester or a surge absorber has been employed in power system and electrical circuits.
A nonlinear resistor, having a nonlinear resistance characteristic at 1 normal voltage but exhibiting a low resistance value during overvoltage, works in such as instrument.
For example, FIG. 19 shows a conventional discharge counter which adopts a carbon silicon (hereinafter referred to as "SiC") element as a nonlinear resistor. The discharge counter includes SiC elements 160a-160c, a capacitor 170, a counter 30, a G1 gap 180a, a G2 gap 180b, and a lightening arrester terminal 40. Between the lightening arrester terminal 40 and a grounding terminal 50, the SiC element 160a and the capacitor 170 are connected in series. The series circuit of the SiC element 160b and the capacitor 170 are connected in parallel with a serial circuit of the G1 gap 180a, the G.sub.2 gap 180b. These circuits are the parallel circuit.
The SiC element 160c is provided between an intermediate electrode 180c and the grounding terminal 50. The capacitor 170, the SiC element 160c and the counter 30 are connected in a parallel circuit.
In the discharge counter constructed as described, the action of the lightening arrester LA causes the discharge current to flow into the discharge counter from the lightening arrester terminal 40. This discharge current flows into the ground through the SiC element 160a, the SiC element 160b and the grounding terminal 50. Then, the capacitor 170 is charged by the voltage generated based on the IR drop between both terminals of the SiC element 160b.
After the charging of the capacitor 170, the counter 30 is operated by discharging all or part of the charged energy of the capacitor 170.
Completion of the charging of the capacitor 170 is judged by the following phenomenon. If the discharge current of the lightening arrester LA is small, the completion of the charging of the capacitor 170 is judged by the crest value. If the discharge current of the lightening arrester LA is large, the completion of the charging of the capacitor 170 is judged by the time of a discharge through a short circuit between the G1 gap 180a and the G2 gap 180b in the rising process of the discharge current.
The crest value means the maximum value of the current or voltage in the surge impulse. For example, both the G1 gap 180a and the G2 gap 180b do not short in the EXAMPLE of a small current. When the current increases, the Gi gap 180a is also discharged by residual voltage of both the G1 gap 180a and G2 gap 180b. The discharge current flows into the ground through the SiC element 160c.
The residual voltage means the voltage remaining between both terminals by restricting the overvoltage during discharging. The residual voltage is determined by both the crest value and the waveform of the discharge current. Even when the current increases, the G1 gap 180a also discharges based on the residual voltage of the SiC 160C.
In the above mentioned discharge counter, because the discharge tolerance of the SiC element, as the nonlinear resistance, is small, there is a possibility that the element is destroyed by momentarily large current. Here, the discharge tolerance means the maximum current of the nonlinear resistance which does not trouble the lightening arrester. Accordingly, the protective gap for a by-pass or the parallel circuit composed of SiC elements is adopted to prevent the destruction of the element.
However, a structure like this presents a problem that the size of the discharge counter gets larger due to the needs of the many elements. Further the residual voltage will be high during momentarily large current flows, because the nonlinear resistance characteristic is poor. For example, the residual voltage of 7000V is generated when a discharge current of 10 KA flows. As a result, the size of the discharge counter could be large to increase the tolerance voltage of each element.
In particular, if the momentary discharge current is over 20 KA, the counter 30 does not function because of insufficiently charged necessary energy in the capacitor due to discharging of the gap. This influences reliability.
Even, if this discharge counter is connected to the lightening arrester of a low voltage class (3 kv, 6 kv: for electric rail car), a high terminal voltage is generated between both terminals of the discharge counter based on the residual voltage of the SiC element when a large current from the lightening arrester flows into the discharge counter. This terminal voltage which is added to the residual voltage generated at the lightening arrester, may surpass the standardized residual voltage of the lightening arrester and may not protect the peripheral apparatus.
Accordingly, instead of the SiC element, a zinc oxide element (hereinafter referred to as "ZnO element") can be adopted in the discharge counter.
However, the size of the ZnO element gets large and requires a large size element (for example, with a diameter of 100 mm) the same as the lightening arrester, though the discharge tolerance is larger than that of the SiC element. The ZnO element as the nonlinear resistance, has the characteristic that the operating starting voltage per a thickness of 1 mm is approximately 200V, which is high. A by-pass resistor will, therefore, be needed to overcome the mentioned problems, but it must be large.
The nonlinear resistance is expected to have the range of 20-150 (v/mm) because of the following reasons:
When the varistor voltage is less than 20 (v/mm), the size of the element will be large to get the preferred residual voltage.
When the varistor voltage is more than 150 (v/mm), improvement of the discharge tolerance characteristic will not be expected because it is impossible to increase the thickness of the nonlinear resistor.
Accordingly, the thickness of the ZnO element should be from 1 mm to 2 mm in order to perform with residual voltage less than 1000V on the V-I characteristic. It is, however, very difficult to adjust the mentioned thickness using the ZnO element.